This invention relates to high power (nonlinear) testing of microwave transistors (DUT) in the frequency and time domain. When the transistor is driven in its nonlinear operation regime, a sinusoidal input electrical signal is deformed and the output signal contains harmonic components, described following Fourier's analysis of periodic non-sinusoidal time quantities. The electrical signals injected at the input of the DUT and extracted at the output can be sampled using signal sampling devices, such as signal couplers (see ref. 1), wave-probes (see ref. 5) or IV probes (see ref. 3) and processed by appropriate signal analyzers (see ref. 6).